Multichannel carrier communication system



Jue 28, 1949. K. G. HoDGsoN ET Al. 2,474,249

MULTICHANNEL CARRIER COMMUNICATION SYSTEM Inu nt r MQW 74, Q- Alto nef 3sheets-sheet 2 n Edu K. G. HODGSON ET AL June 28, 1949.

MULTICHANNEL CARRIER COMMUNICATION sYsTEM Filed May 25, 1947 llorneaJune 28, 1949. K, G HQDGSQN ET AL 2,474,249

MULTICHANNEL CARRIER COMMUNICATION SYSTEM Filed May 23, 1947 3Sheets-Sheet 3 [n v nt r I 52W iw@ A Homey Patented June 28, 1949 UNITEDSTATES ATENT OFFICE MULTICHANNEL CARB/IER COMMUNICA- TION SYSTEMApplication May 23, 1947, Serial No. 750,148 In Great Britain April 8,1946 Section 1, Public Law 690, August 8, 1946 Patent expires April 8,19.66

2 Claims. l

The present invention relates to multi-channel signalling systems.

In connection with such a system, which comprises a plurality ofchannels say a channelsof equal or approximately equal frequencyspacing, it is known to derive each of a number-say ,S-of groups of nchannels by a first modulation process, so that a=n, each group ofchannels occupying the same primary frequency range, and subsequently totranslate, by means of a second modulation process, the groups to theirfinal different positions within the frequency range to be transmitted.If the primary frequency range lies within the frequency range which isnally transmitted, one group, hereinafter called the basic group may beused without a second modulation process, and this group is notnecessarily the lowest group in the frequency range to be transmitted aswill be made clear in the following. In this case, however, it isdifficult to avoid interference between the basic group and the groupsimmediately adjacent to the basic group. The reason for thisinterference is that the output of the second modulator contains thewanted and unwanted sidebands and also the input frequencies which liein the frequency range of the basic group. In most of the groups theunwanted sidebands and the input frequencies can easily be eliminated byfilters, but in the groups adjacent to the basic group the frequencyspace between the wanted sideband and the band in which the inputfrequencies lie is so small that filters have to be used of an extremelysharp cut-off characteristic in order to eliminate the input bandwithout introducing distortion into the wanted sideband. Even if themodulators are balanced to reduce the transmission of the inputfrequency, the'lter requirements are very difficult to meet. In order toovercome this difficulty, one could provide a frequency gap between thebasic group and the adjacent groups. This, however, would reduce thenumber a of transmission channels within the available total frequencyrange.

It is an object of the invention to provide a multi-channel signallingsystem which overcomes these difficulties without the necessity ofproviding substantial frequency gaps within the total frequency range oftransmission.

A multi-channel signalling system, according to the invention, employs abasic group of channels occupying a primary frequency range, othergroups of channels each occupying the whole or part of the primaryfrequency range, and comprising means for translating such other groupsto frequency ranges outside the primary frequency range and ischaracterised in this that groups of channels which are translated tooccupy positions in the frequency range adjacent to the primaryfrequency range have bandwidths which are less than the whole bandwidthof the primary frequency range.

To make the invention more clearly understood, reference will now bemade to the accompanying drawings, which are given by way of exampleonly and in which:

Figs. 1 and 2 serve to illustrate the underlying principle of theinvention;

Figs. 3 and 4 illustrate arrangements of known multi-channel alternatingcurrent telegraph systems;

Figs. 5, 6, and '7 serve to illustrate the principle of the invention asapplied to the arrangement shown in Fig. 4.

Referring rst to Fig. 1, the arrow I indicates a basic group of nchannels in the primary frequency range from f I to f2. The arrow 2indicates another group of nchannels, extending from the frequency 'flto f3. By modulating this other group with the frequency (f2-H3), asindicated at 3, and employing the lower sideband only, the said othergroup is translated to assume a frequency range extending from f2 to(f2-i-J3-fl), as indicated at 4. It will be seen that anuninterruptedfrequency range is utilised without any substantial gap, that is to say,the frequency range from f! to f2 is occupied by the basic group I andthe range from f2 to (f2 -|f3-fl') is occupied by said other group afterit has been translated to the position 4 (the groups I and 4 which areactually transmitted being indicated by arrows in full lines), while yetthe requirements on the design of filters are considerably eased sincethe separation of the frequencies between f3 and f2 from the band 4after the process of ymodulating the band 2 with the carrier 3 is notcritical; in other Words, the frequency gap between the input band 2 andthe translated band 4 allows the filter attenuation to rise to a valuenecessary to eliminate the input frequency band 2 from the translatedband 4, so that the input band 2 can no longer interfere with the bandI.

If the total frequency band available for transmission extends to lowerfrequencies beyond fl, a third group 5 of n-y channels, extending fromthe frequency f4 to the frequency f2, may be modulated by the frequency(f4-HI) indicated at 1, whereby the group, by using the lower sideband,is translated to the frequency band 6, ex-

tending from fl to (fd-l-fI-f2). It will, again, be realised that afrequency gap extending from f I to f4 exists between the inputfrequency band 5 and the translated band 6 which is being transmitted.Again, this gap is sufficient to allow the filter attenuation to rise toa value necessaryrto eliminate the input frequency band 5, while thetransmitted band 6 is adjacent to the basic band I, without leaving anygap in the frequency range to be transmitted.

The transmitted groups of frequency bands are now I, 4, and 6 (indicatedby arrows in full lines) and cover an uninterrupted frequency range.

In the example illustrated, in Fig. 1 the groups 4 and 6, adjacent tothe basic group I, each comprise fewer channels than the basic group I.If' it is desired to transmit groups of channels of equal widths, thesystem so far described may be modified as will now be explained withreference to Fig. 2. In Fig. 2, the basic group of n channels is againindicatedby the arrow I. Another group of n channels, which eventuallyoccupies the upper frequency range adjacent to= the basic range If, isbroken up so as to form twosub-groups, one comprising '1t-x channels andextending from frequency fl to f3 as indicated at 2, and the othercomprising r channels and extending from f3- to f2 as indicated at t0.The frequencies of the subgroup 2 are modulated by the frequency (f2-H3)as indicated at 3, and the sub-group I'Il is modulated by the frequency(2]2-1-33- f I as indicated at I I. lf lower sidebands are used, thesubgroup I is translated to assume the finaly position 'tV as in Fig. 1,and the sub-group I9 is translated to assume the final position I2. Itwill be seen ythat the groups I, 4 and I2 occupy an uninterruptedfrequency range, while sufficient gaps exist between the translatedgroup 4 and the input group 2 on the one hand and the translated grouplf2 and the input group Ill on the other, for easing the filter design.

A third group of n channels may be broken up into two sub-groups l'3-and I4 comprising respectively n-y and y channels, and extending fromf4to f2 and from JIl to f4 respectively. The group I3 is modulated by thefrequency (f4-HI) as indicated at I 5, and the group I 4 by thefrequency (2fI-l-f4-f2) as indicated at I6. If, again, lower sidebandsare used, the sub-groups I3 and I4 are translated respectively tothepositions Il and I8, extendingifrom (fI|-f4-f2) to fI and from(2H-f2) to fI-l-.f4-f2). Again, it will be appreciated that the groupsI8, II, I, 4, and I2 (indicated by arrows in full lines) occupy It is,however, possible to use upper sidebands in the translating operationand suppress the lower sidebands, the modulating frequency used for thetranslation being the difference of the two boundary frequencies of thebasic group. Since the modulating frequency in this latter case would becomparatively low, it is usually preferable to employ the lowersidebands.

The principle of the invention may be used in connection with multiplechannel carrier telephony and with multi-channel Voice frequencytelegraphy. It is also obvious that the invention isx not restricted` tosystems using two subsequent modulations but may with equal advantage beused in connection with systems using more than two such modulations.

Applications of the invention to multi-channel voice frequency telegraphsystems will now be described. Fig. 3 diagrammatically illustrates such.a system of known type arranged for unidirectional operation. For thereverse direction, the same frequencies are used along a secondcommunication channel with equipment at the sendingl and the receivingends similar to those indicated in Fig. 3. At the transmitting side, theequipment for one channel comprises a telegraph modulator MI formodulating a frequency fl with direct current telegraph signals, abandpass filter BFI which transmits the frequency fl' with the telegraphsidebands, and at the receiving side a bandpass filter BFIy and adetector DI for converting the alternating current signals for operatingthe telegraph equipment, the transmitting and receiving sides beingconnected by any suitable transmission Vpath such as a cable. Theequipment for the channels 2, 3 n is similar to that of channel I, theoutput of all the transmitting bandpass filters being connected inparallel to one another and. to the line, and likewise the inputs of allthe receiving bandpass filters being connected. in parallel to oneanother and4 to the line. With such a system it is usual to choose thefrequencies fl to fn, so that they are odd harmonics of a singlefundamental frequency. Thus, the telegraph channels are spaced uniformlyin the total frequency range available for transmission. Fig. 5represents the attenuation frequency characteristics of the sending orreceiving band filters BFI to BFn.

The design of the lters BFI to BFV', becomes increasingly difficult asthe frequency increases. For example. in practice it has been possibleto arrange the frequencies fI to fn at a spacing of 120 C. P. S. withinthe frequency range 420 to an uninterrupted frequency range availablefor 2460 C; P. S. the latter being 0105 130 the 11131061 transmissionwithout any substantial gap, while gaps exist between each input groupand the corresponding translated group whereby each input group may besuppressed from the translated limit (2500 C. P. S.) of the band widththat has been heretofore used for telephony. Wider bandwidths, up to3500 C. P. S. are now, however, being introduced to an increasing extentand it is group without raising unduly the requirements desirable. ifone of the channels of a multiplex of the lters.

In the examples so far described, only the lower side bands are employedand the upper sidebands suppressed, and the frequency with which a groupof channels is modulated is the sum of the frequencies representingone-boundary of the group that is to be translated, and thecorresponding boundary of the translated group. This means` it will beseen, that when a boundary of the translated group is the same as oneboundary of the basic group the group before translation occupies apartof the frequency range of the basic group` and the carrier frequencyusedfor translation is the sum ofthe frequencies at either carriertelephone system is to be allotted for the provision of a multiplexvoice frequency telegraph system, that the bandwidth available should befully occupied by telegraph channels. The extension of the upperfrequency range of known systems means increasing difficulty in filterdesign to the point of' impracticability if the existing channel spacingis to be retained. The obvious remedy would be to depart from theuniform spacing of the signal channels and to increase the frequencyspacing in the upper frequency range available for transmission. This,however would prevent the derivation from a single fundamental frequencyof all the frequencies fl to fn unless end of the gap between the twofrequency ranges. the spacing is increased by the width of a channel ora multiple thereof, which would considerably reduce the number ofchannels otherwise available for transmission. This difficulty isovercome by means of the present invention.

In the arrangement shown in Fig. 4, a basic group of n channels isderived by direct modulation by means of modulators MI Mn and bandpassfilters BFI BFn, the modulated frequencies being demodulated bydetectors DI Dn. Another group of n channels is derived by modulatorsM1I M111. and bandpass filters BFlI BFln and a second modulation ofchannel frequencies fl to fn by means of group modulator GM using acarrier frequency fx. GBF represents a group band filter and Prepresents coupling means to separate the basic group of channels fromthe other group of channels. At the receiving end, similar couplingmeans P separate the basic group of channels from the group modulatedchannels, which pass through the group band filter GBF to the groupdemodulator GDM using the same carrier frequency faz. The system so fardescribed is also known.

Fig. 6 shows the attenuation frequency characteristics of the bandfilters BFI to BFn and the characteristic of the group band filters GBF.The vertical characteristics of the band filters BFII to BFln when groupmodulated and shifted in the frequency range by the frequency fa: areshown in dotted lines. The requirements of the group band lter GBF atthe sending end are that this filter shall pass one set of sidebandsderived from modulation of frequencies fI to fn by the carrier frequencyfri: with a minimum attenuation, whereas it shall attenuate the inputfrequencies fI to fn sufficiently to avoid interference between the saidinput frequencies and the primary frequencies f I to fn of the basicgroup of channels which have not undergone a second modulation. At thereceiving end the requirements of this filter are similar. It isdifficult or impossible to design a lter to achieve this purpose withoutallowing a gap within the frequency range actually transmitted betweenthe channel n of the basic group of channels and the adjacent channel ofthe group modulated channels, and at least one or possibly two such-channels would be lost in the frequency spectrum available fortransmission.

The invention is applied as shown in Fig. '7. As in Fig. 6 a basic groupis derived by direct modulation by means of modulators MI Mn and bandpass filters BFI BFn. A second group of channels fl to fn, where fm islower than fn is derived by modulators M1I Mlm and band pass filtersBF1I BFlm. This group is then translated to the position in thefrequency spectrum adjacent to the basic group by a second modulation bymeans of group modulator GM using a carrier frequency fx=fn|fm. Thegroup band filter GBF is used to select the lower sideband and tosuppress the carrier frequency and the upper sideband. The lowersideband is applied to the line through a coupling device P such as ahybrid coil.

A similar result would be obtained by using for the carrier supplied tothe modulator GM, a frequency of fit-fl, and selecting the uppersideband by the filter GBF. As stated above, however, it is preferred touse the lower sideband.

The requirements of the group band filter GBF are illustrated Fig. 8,from which it may be seen that it is now permissible for the frequenciesbetween f(m-{-1) and fn of the basic group which are not group modulatedto lie within the pass range of GBF without suiereing interference fromthe group modulated channels, thus allowing uniform spacing of thefrequencies throughout the frequency range available for transmission bychoosing f=fm+fn and using the lower sideband. The same argument isvalid for the receiving end where frequencies between f(m|.l) and fn areaccepted by the receiving group band filter GBF but are not accepted bythe receiving band filters BF1I to BF1m after demodulation.

If the line between transmitting and receiving stations is capable oftransmitting frequencies as high as 2m, then a number of channels equalto n-m are formed by modulation in modulators M1(m+ 1) to Mln by carrierfrequencies f(m+ 1) to fm. This sub-group is group modulated in groupmodulator G1M with a frequency fy=2fnlfmfl, the lower sideband isselected by a filter GBFl and applied to the line through a couplingdevice P1 separating this sub-group from the basic group and theside-band passing the filter GBF. It will be seen from the foregoingdescription of Fig. 2, that the actually transmitted frequency range isuninterrupted and yet the requirements of the filters are considerablyeased. Furthermore, it is possible to have a basic group of channelsderived directly from primary frequencies fl to fn, another group ofchannels derived from input frequencies f I to fm where fm is less thanfn by a single group modulation as described whereby a block of channelsof total number nH-m is obtained. Similarly, a second block of channelsof total number 11H-p may be obtained wherein fp is less than fn and thetwo blocks may be combined with each other in accordance with thisinvention so as to result in a uniform channel frequency spacing on thetransmission line throughout the range of frequencies which are passedby the line.

Furthermore, the means by which a group or groups of channels is or aretranslated to positions in the frequency spectrum below the basic groupwill be clear. From the foregoing and from a comparison with Figs. 1 and2, the only alteration necessary being in the choice of carrierfrequency to be used in the group modulator or modulators. It is alsoclear that, although the invention has been illustrated as applied totelegraph channels it is equally applicable to carrier telephone orother communication channels systems.

What is claimed is:

1. A multi-channel signalling system comprising modulating means andfilter means for creating a basic group of channels occupying a primaryfrequency range and impressing said group on a transmission line,modulating means and filter means for creating another group of channelsoccupying a part of said frequency range from one boundary thereof to anintermediate frequency, group modulating means for modulating said othergroup of channels with a frequency equal to the sum of said intermediatefrequency and the frequency of the other boundary of said range, filtermeans for selecting the lower side band from the output of said groupmodulating means and means for impressing said lower side band on saidline.

2. A multi-channel signalling system comprising modulating means andfilter means for creating a basic group of channels occupying a, primaryfrequency range and impressing said group on a transmission line,modulating means and filter means for creating another group of channelsoccupying a part ofv said frequency range from one boundary thereof toan intermediate REFERENCES CITED frequency, group modulating means formodulat- The following references are of record in the ing said othergroup of channels with a frequency file of this patent:

equal to the diierence between said boundaries of said primary frequencyrange, filter means for UNITED STATES PATENTS selecting the uppersideband from the output of Number Name Date said group modulatingmeans, and means for im- 2,270,385 Skillman Jan. 20, 1942 pressing saidupper sideband on said line. 2,274,535 Dixon Feb. 24, 1942 KENNETHGEORGE HODGSON. 10 FRANK FAIRLEY.

